1. Technical Field
The present invention relates to mold blocks, and more particularly to mold blocks which circulate about an endless path to vacuum form corrugated tubes.
2. Background Art
A well known method for manufacturing corrugated pipes is to extrude a tube of thermoplastic from a head and then conform that tube to the interior of a corrugated mold tunnel formed by mold blocks. The mold blocks typically come in pairs and mate together to form a portion of the mold tunnel, and the thermoplastic is either blown into its shape (by pressure created within the mold tunnel) or by vacuum (where vacuum draws the air from around the mold tunnel).
Structures which have been used in the vacuum method of manufacture are illustrated in U.S. Pat. Nos. 4,319,872, 4,439,130, and 4,718,844. These patents all address the problem of drawing a uniform, strong vacuum around the entirety of the mold blocks which are continually moving during the molding process. As can be seen from these structures, it is difficult to inexpensively form any such mold block.